I'm having a really hard time holding threat when tanking against more than 4 monsters in Naxx. I don't know if it's the way I approach tanking. But our usual main warrior tank has no such problems. This is really a problem when tanking the live side in the heroic Gothik fight. I usually start losing aggro during the 2nd horseman spawn. Any sort of tips/help is really appreciated.
Just to the left of where you enter the room is a corner. All range should stand there. You stand between them and the three doors. Basically, all mobs are forced to come by you. From there, just judiciously time your TCs and shockwaves and you should be fine.
Mind you I've only been doing it in 10-man, 25 might be more interesting. But with the extra tank(s) on each side, probably not.